In many telecommunication systems, the individual data frames of a number of channels are generated asynchronously, i.e. ATM data frames are not generated aligned to a given synchronization clock in the transmitter. If the individual channels have the-same basic transmission rate TRB (on the air interface) the data frames will contain the same number of data symbols, however, they will still not be aligned to a common synchronization clock. The task of the transmitter is, despite the asynchronously arriving data packets from the individual channels, to transmit the data frames (or more precisely their data symbols) on the radio link with a predetermined respective time-offset per channel, which then characterizes the beginning of data frames of this particular user channel. The transmission to the receiver can introduce further time delays between the individual data frames, for example due to varying distances during the transmission, such that the receiver must perform a time-alignment with respect to a common synchronization clock provided in the receiver. The present invention relates to the time-offset adjustment in the transmitter.
An example of a mobile radio communication system, where such a time-offset adjustment is required, is a CDMA mobile radio communication system. FIG. 5 shows a block diagram of a base transceiver station BTS in such a CDMA communication system. FIG. 6 shows the encoder unit ENC of the base transmitter station BTS in FIG. 6. It should be noted that hereinafter the invention and their problems will be considered with respect to the CDMA system shown in FIG. 5, 6, however, the time-alignment is also applicable to any communication system requiring a time-offset adjustment.
In all telecommunication systems, where several channels each comprising successive data frames are provided, a separate encoder dedicated to a specific user channel must be provided in order to encode the successively arriving data frames of one particular channel. However, this would result in e.g. up to 300 encoder units, which is unacceptable in terms of the required hardware effort. For this reason, there is always the problem of how a common encoder resource can be used efficiently for encoding the data frames of all channels. In principle, this can be achieved if the time, which the encoder spends on encoding one data frame is much shorter than the duration of the data frame itself. Then, the encoder can process the data frames of several channels one after the other within one data frame period. Since the data frames of the individual channels do not arrive synchronized to a common synchronization clock and must be provided with a predetermined time-offset per channel to the air interface, the arriving data frames must be buffered in a memory, before a predetermined time-offset can be applied to them such that the data symbols can be delivered in form of a constant stream of data symbols to the modulation unit (e.g. the CDMA modulator BBTX in FIG. 6).
The invention particularly relates to the problem of how different time-offsets can be applied to the (possibly asynchronously) arriving data frames of a great number of channels.